This invention relates to electric hammers having an air cushion hammering mechanism.
Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are generally provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Some hammers can be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. A rotary drive mechanism transmits rotary drive from the electric motor to the spindle to cause the spindle, or a forwardmost part thereof to rotate.
The spindle of a hammer generally requires axial stops to be located on it for limiting the axial movement, with respect to the spindle of components which are located both within the hollow spindle and mounted around the hollow spindle.
In known designs of hammer, when the hammer is to be used the forward end of a tool or bit is pressed against a workpiece, which urges the tool or bit rearwardly within the hammer spindle. The tool or bit in turn urges the beatpiece rearwardly into its operating position in which the rearward end of the beatpiece is located within the reciprocating path of the ram. In the operating position the beatpiece receives repeated impacts from the ram. When the hammer is in use, the forward impact from the ram is transmitted through the beatpiece to the bit or tool and through the bit or tool to the workpiece. A reflected impact is reflected from the workpiece and is transmitted through the bit or tool to the beatpiece. This reflected, or reverse impact must be absorbed within the structure of the hammer in such a way that the reverse impacts do not over time destroy the hammer and so that the reverse impacts are not transmitted to the end user.
When the user takes the tool or bit of the hammer away from the workpiece, the next forward impact of the ram on the beatiece urges the beatpiece forwardly into its idle mode position. The beatpiece can move forwardly and stay forwardly because the tool or bit is no longer urging it rearwardly, as the tool or bit can now itself assume a forward idle mode position. Because the beatpiece does not now offer much resistive force against the ram, the ram can also move into a forward idle mode position. In the idle mode position of the ram, the air cushion is generally vented and so any further reciprocation of the piston has no effect on the ram. This forward movement of the components on entry into idle mode generates the greatest impact forces on the structure of the hammer, in particular on the hammer spindle. This is because the forward impact force of these parts on entry into idle mode is not transferred to the workpiece, but has to be absorbed by structure of the hammer itself. Thus, the number of idle strikes, ie. the number of reciprocations of the ram, beatpiece and tool or bit, when the bit or tool is removed from the workpiece need to be minimised in order to minimise the number of high impact force idle strikes that have to be absorbed by the structure of the hammer. This can be achieved by catching the ram and/or the beatpiece in their idle mode positions so that they cannot slip rearwardly to cause the ram to move into a position in which the air cushion is closed and the ram and thus the beatpiece begin to reciprocate again.
Axial stops for limiting forward and rearward movment may be required for components within the spindle, such as a beatpiece catching or ram catching arrangement or a beatpiece guiding arrangement. Axial stops for limiting forward movement may be required for components which transfer idle mode impacts from components within the spindle to the spindle on entry into idle mode. In addition, axial stops for limiting rearward movement may be required for components which transfer reflected impacts from the beatpiece to the spindle during normal operation of the hammer.
Axial stops may also be required for components which are mounted around the spindle. In known designs of rotary hammer an axially moveable spindle drive sleeve or gear may be mounted around the spindle. In a first axial position the sleeve or gear transfers rotary drive from an intermediate drive shaft to the hollow spindle, or a forward part of the hollow spindle and in a second axial position the sleeve or gear does not transfer said rotary drive. The axial position of the spindle drive sleeve or gear is selected via a mode change mechanism actuated by a mode change knob. Axial stops may be required to set the end positions for the axial movement of the spindle drive sleeve or gear. In known designs of rotary hammer, an overload clutch may be mounted around the spindle in association with a spindle drive sleeve or gear for transmitting torque to the spindle only below a predetermined torque threshold. The overload clutch may be loaded by a helical spring which spring is mounted around the spindle and an end stop may be required as a surface against which the spring bears in order to bias the clutch into an engaged position. Known arrangements for enabling a tool holder spindle portion to be removed from or fitted to or rotated with respect to a main spindle portion will comprise components mounted around the spindle which may require axial stops.
Axial stops for components located within the hammer spindle are generally formed by forming the internal surface of the hollow cylindrical spindle so that it has a stepwise increase in its internal diameter, in the axial direction, from the front to the rear of a spindle component part in order to generate one or more annular rearward facing shoulders within the spindle. The annular shoulders can act as axial stops to limit the forward movement of components located within the spindle. Within a single spindle part the internal diameter of the spindle cannot increase and then decrease, as this would make it difficult or impossible to assemble components within the increased internal diameter portion of the spindle. It is generally preferred that the front end of the spindle has the smallest internal diameter as the diameter of the tool or bit, which is to be fitted therein, generally has a smaller diameter than the diameter of the piston and ram which are located within the rearward portion of the spindle. It should be noted also that a simple spindle structure is preferred with the spindle formed from a single component part or in two parts with a forward tool holder portion of the spindle removeable, so that tool holders can be removed and replaced.
Thus, the annular shoulders are able to provide axial stops against forward movement of components within the spindle, but cannot provide axial stops against rearward movement within the spindle. The general solution for limiting rearward axial movement of components located within the spindle is by the use of metal circlips. The circlips have a generally circular radial cross-section, part of which is received in a corresponding annular groove formed in the internal surface of the spindle, at the desired axial stop location, so that the remaining part of the circlip extends radially inwards beyond the internal surface of the spindle. Thus, the circlip can form an axial stop.
The problem with circlips is that they are difficult to correctly assemble into the hammer spindle. If the circlip is not correctly assembled then the axial stop is not effective and the hammer will not operate correctly. Also, if the circlip is not correctly assembled it is likely to come loose and this is likely to cause damage to the hammer when it is first used.
Alternatively, axial stops for limiting rearward axial movement can be formed by using several separate spindle parts to form the hollow cylindrical spindle, which spindle parts have differing adjacent internal diameters or which spindle parts have other components extending between the separate spindle parts to form end stops. The use of multiple spindle components adds complexity and makes it difficult to seal the interior of the spindle from the ingress of dust.
Similarly, stepwise increases in the external diameter of the spindle can be used to provide annular forward facing shoulders which act as stops for limiting axial rearward movement of components which are mounted around the spindle. Circlips mounted within cooperating grooves formed within the external surface of the spindle or multiple spindle parts are generally used to form axial stops for limiting the axial forward movement of components mounted around the spindle, with the disadvantages set out above.